Gallium nitride (GaN) offers substantial opportunity to enhance performance of electronic devices such as high electron mobility transistors (HEMTs). The HEMT behaves much like a conventional Field Effect Transistor (FET), and the fabrication of HEMT devices is based on FET architecture. However, HEMTs require a very precise, lattice-matched heterojunction between two compound semiconductor layers. In general, a GaN HEMT has a Schottky layer and a GaN buffer layer deposited on a substrate and source, gate, and drain contacts deposited on the Schottky layer.
The GaN-based HEMT device is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between the AlGaN layer, which has a large band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional electron gas in the undoped GaN layer. The amount of current is controlled by applying voltage to the gate electrode, which is in Schottky contact with the semiconductors so that electrons flow along the channel between the source electrode and the drain electrode.
As the market for HEMTS continues to grow, many improvements remain desirable to enhance various operating characteristics such as the breakdown voltage Vbr and the leakage current I. For example, one problem that remains to be adequately addressed arises because the Schottky layer is typically metallic and may be exposed to air during fabrication of the HEMT and/or during operation of the HEMT. By exposing the Schottky layer to air, surface reactions such as oxidation may occur on the surface of the Schottky layer. These surface reactions may degrade the performance of the HEMT and also decrease the effectiveness of passivation. Passivation is the deposition of a dielectric material on the surface of the HEMT in order to passivate, or fill, surface traps on the surface of the HEMT, thereby avoiding device degradation due to these surface traps such as RF to DC dispersion.
Therefore, there remains a need for a high voltage GaN HEMT structure that, among other things, has a reproducible termination layer capable of preventing surface reactions during fabrication and operation of the GaN HEMT.